
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.TreeSet;

public class C1_11_9 {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String tmp = br.readLine();
        double L = Double.valueOf(tmp.split(" ")[0]);
        double n = Double.valueOf(tmp.split(" ")[1]);
        double c = Double.valueOf(tmp.split(" ")[2]);
        double LL = (1+n*c)*L;
        double left = 0;
        double right = Math.PI;
        double mid = 0;
        while(right - left >= 1e-12)
        {
            mid = (left + right) / 2;
            if(L*mid/(2*Math.sin(mid/2)) < LL)
               left = mid;
            else
               right = mid;
        }                         
        System.out.printf("%.3f", LL/mid*(1-Math.cos(mid/2)));
    }
}
